The problem of detecting the presence of a trailing vehicle in the blind spot of a vehicle is well known. For example, U.S. Pat. No. 5,122,796 issued Jun. 16, 1992 to George Beggs et al and assigned to Auto-Sense Ltd., discloses an active system using an electro-optic emitter and receiver. The emitter operates in a short wavelength infrared region thereby avoiding interference from long wavelength infrared or thermal emitted radiation sources such as hot objects. A band pass filter is used to limit the detector to the pass band of the emitter.
For add-on devices in automotive applications, it is desirable to operate with power sources independent of the main vehicle power system. Active devices such as the Beggs et al emitter/detector system have a relatively high power consumption and may need connection to the vehicle's electrical system.
Passive detector systems have been proposed. For example, the present inventor is a co-inventor of a PCT International Patent Application filed Mar. 10, 1995, proposes using a thermal emitted radiation detector of the type found in security system motion detectors for a passive blind spot detector. This type of thermal emitted radiation detector requires a change in temperature to provide an output.
As in the motion detector application, the proposed detector relies upon movement of the heat-generating object through the field of view of the detector to generate an output signal. This type of detector is, however, insensitive to relatively stationary objects. Hence, a trailing vehicle, remaining in the blind spot, if remaining in a fixed relative position, becomes undetectable and may be forgotten by the vehicle operator.